In recent years, various types of optical recording have been investigated in the field of information recording. In the optical information recording system, recording/reproducing can be made at high density and low cost with a non-contact mechanism, and its applications have now become widespread. At present, for example, an available optical disk has a structure in which an information layer is formed on a transparent resin substrate with a thickness of 1.2 mm and protected by an overcoat; or a structure in which two transparent resin substrates each with a thickness of 0.6 mm and an information layer provided on one or both sides of the substrate are bonded to each other.
In recent years, the way to increase the recording density of the optical disk has been investigated, for example, including increasing a numerical aperture of objective lens and shorting a wavelength of a laser beam. If the substrate on the recording/reproducing side (the substrate on the incident laser beam side) has a relatively small thickness, the laser beam spot can be less affected by aberration, and therefore, the disk can have a relatively large tolerance for the inclination angle (tilt). From this point of view, it has been proposed that the recording/reproducing side substrate should have a thickness of 0.1 mm, the NA be about 0.85 and the laser beam have a wavelength of about 400 nm. Under such conditions, variations in thickness of the recording/reproducing side substrate should preferably be controlled to 5% or less, in terms of the influence of recording/reproducing light on focus or spherical aberration.
A potential way to control the variations in thickness to such a small value may include bonding substrate sheets each having a uniform thickness of a few tens of μm to each other with a radiation cure type resin. However, the process using such substrate sheets is very expensive. Therefore, a spin coating method should preferably be used to form the base material only from a radiation cure type resin on the recording/reproducing side.